Directories Searched by the Runtime Linker

By default, the runtime linker looks in only one standard place for dependencies: /usr/lib for 32-bit dependencies, or /usr/lib/64 for 64-bit dependencies. Any dependency specified as a simple file name is prefixed with this default directory name and the resulting path name is used to locate the actual file.

The actual dependencies of any dynamic executable or shared object can be displayed using ldd(1). For example, the file /usr/bin/cat has the following dependencies:

$ ldd /usr/bin/cat
        libc.so.1 =>     /usr/lib/libc.so.1
        libdl.so.1 =>    /usr/lib/libdl.so.1

The file /usr/bin/cat has a dependency, or needs, the files libc.so.1 and libdl.so.1.

The dependencies recorded in a file can be inspected by using the dump(1) command to display the file's .dynamic section, and referencing any entries that have a NEEDED tag. In the following example, the dependency libdl.so.1, displayed in the previous ldd(1) example, is not recorded in the file /usr/bin/cat. ldd(1) shows the total dependencies of the specified file, and libdl.so.1 is actually a dependency of /usr/lib/libc.so.1.

$ dump -Lvp /usr/bin/cat
 
/usr/bin/cat:
[INDEX] Tag      Value
[1]     NEEDED   libc.so.1
.........

In the previous dump(1) example, the dependencies are expressed as simple file names. In other words, there is no `/' in the name. The use of a simple file name requires the runtime linker to generate the required path name from a set of rules. File names that contain an embedded `/' will be used as provided.

The simple file name recording is the standard, most flexible mechanism of recording dependencies. The -h option of the link-editor records a simple name within the dependency. See "Naming Conventions" and "Recording a Shared Object Name".

Frequently, dependencies are distributed in directories other than /usr/lib or /usr/lib/64. If a dynamic executable or shared object needs to locate dependencies in another directory, the runtime linker must explicitly be told to search this directory.

The recommended way to indicate additional search paths to the runtime linker is to record a runpath during the link-edit of the dynamic executable or shared object. See "Directories Searched by the Runtime Linker" for details on recording this information.

Any runpath recording can be displayed using dump(1) and referring to the entry that has the RUNPATH tag. In the following example, prog has a dependency on libfoo.so.1. The runtime linker must search directories /home/me/lib and /home/you/lib before it looks in the default location /usr/lib.

$ dump -Lvp prog
 
prog:
[INDEX] Tag      Value
[1]     NEEDED   libfoo.so.1
[2]     NEEDED   libc.so.1
[3]     RUNPATH  /home/me/lib:/home/you/lib
.........

Another way to add to the runtime linker's search path is to set the environment variable LD_LIBRARY_PATH. This environment variable, which is analyzed once at process startup, can be set to a colon-separated list of directories. These directories will be searched by the runtime linker before any runpath specification or default directory.

These environment variables are well suited to debugging purposes, such as forcing an application to bind to a local dependency. In the following example, the file prog from the previous example is bound to libfoo.so.1, found in the present working directory.

$ LD_LIBRARY_PATH=. prog

Although useful as a temporary mechanism of influencing the runtime linker's search path, the use of the LD_LIBRARY_PATH environment variable is strongly discouraged in production software. Any dynamic executables that can reference this environment variable will have their search paths augmented. This augmentation can result in an overall degradation in performance. Also, as pointed out in "Using an Environment Variable" and "Directories Searched by the Runtime Linker", the LD_LIBRARY_PATH environment variable affects the link-editor.

A process can inherit an environment such that a 64-bit executable is given a search path that contains a 32-bit library matching the name being looked for, or vice versa. The runtime linker then rejects the mismatched 32-bit library and continues down its search path looking for a valid 64-bit match. If no match is found, an error message is generated. This can be observed in detail by setting the LD_DEBUG environment variable to include the files token. See "Debugging Library".

$ LD_LIBRARY_PATH=/usr/bin/64 LD_DEBUG=files /usr/bin/ls
...
00283: file=libc.so.1;  needed by /usr/bin/ls
00283: 
00283: file=/usr/lib/64/libc.so.1  rejected: ELF class mismatch: \
00283:                                  32-bit/64-bit
00283: 
00283: file=/usr/lib/libc.so.1  [ ELF ]; generating link map
00283:     dynamic:  0xef631180  base:  0xef580000  size:      0xb8000
00283:     entry:    0xef5a1240  phdr:  0xef580034  phnum:           3
00283:      lmid:           0x0
00283: 
00283: file=/usr/lib/libc.so.1;  analyzing  [ RTLD_GLOBAL  RTLD_LAZY ]
...

If a dependency cannot be located, ldd(1) indicates that the object cannot be found. Any attempt to execute the application results in an appropriate error message from the runtime linker:

$ ldd prog
        libfoo.so.1 =>   (file not found)
        libc.so.1 =>     /usr/lib/libc.so.1
        libdl.so.1 =>    /usr/lib/libdl.so.1
$ prog
ld.so.1: prog: fatal: libfoo.so.1: open failed: No such file or directory